US group separates on-chip optics and electronics

24 Apr 2018

MIT, Berkeley and BU team integrates components into existing chips enabling optimization of photonics away from electronics.

A team of researchers led by groups at MIT, the University of California at Berkeley, and Boston University have announced what they are calling “another milestone” in electro-optical chip development.

This is a new technique for assembling on-chip optics and electronic separately, which enables the use of more modern transistor technologies. New it may be, but the technique requires only existing manufacturing processes.

The latest breakthrough, reported in Nature, follows the group’s 2015 fabrication of a previous generation microprocessor, constructed using only existing manufacturing processes, which integrated electronic and optical components on the same chip.

At that time, the researchers’ approach required that the chip’s electrical components be built from the same layer of silicon as its optical components. That meant relying on an established chip technology in which the silicon layers for the electronics were thick enough for optics.

‘Optimization’

“The most promising thing about this work is that you can optimize your photonics independently from your electronics,” commented Amir Atabaki, a research scientist at MIT’s Research Laboratory of Electronics and one of three first authors on the new paper.

“We now have different silicon electronic technologies, and if we can just add photonics to them, it would be a great capability for future communications and computing chips. For example, now we could imagine a microprocessor manufacturer or a GPU manufacturer like Intel or Nvidia saying, ‘We can now have photonic input and output for our microprocessor or GPU.’ And they would not need to change much in their process to get the performance boost of on-chip optics.”

Moving from electrical communication to optical communication is attractive to chip manufacturers because it could significantly increase the chips’ speed and reduce power consumption, an advantage that will become more significant as chips’ transistor counts continue to rise. The Semiconductor Industry Association has estimated that at current rates of increase, computers’ energy requirements will exceed the world’s total power output by 2040.

The integration of optical and electronic components on the same chip reduces power consumption still further. Optical communications devices on the market today, says Atabaki, "consume too much power and generate too much heat to be integrated into an electronic chip such as a microprocessor". A commercial modulator, which encodes digital information onto the light signal, consumes between 10 and 100 times as much power as the modulators built into the researchers’ new chip.

It also takes up 10 to 20 times as much chip space. The integration of electronics and photonics on the same chip enables Atabaki and his colleagues to use a more space-efficient modulator design, based on a photonic device called a ring resonator.

“We have access to photonic architectures that one cannot normally use without integrated electronics,” Atabaki added. “For example, today there is no commercial optical transceiver that uses optical resonators, because you need considerable electronics capability to control and stabilize that resonator.”

Atabaki’s co-first-authors on the Nature paper are Sajjad Moazeni, a PhD student at Berkeley, and Fabio Pavanello, who was a postdoc at the University of Colorado at Boulder, when the work was done. The senior authors are Rajeev Ram, a professor of electrical engineering and computer science at MIT; Vladimir Stojanovic, an associate professor of electrical engineering and computer sciences at Berkeley; and Milos Popovic, an assistant professor of electrical and computer engineering at Boston University.

Sizing crystals

The earlier work on integrated photonics, which was also led by Ram, Stojanovic, and Popovic, involved a process called wafer bonding, in which a single, large crystal of silicon is fused to a layer of glass deposited atop a separate chip. The new work, in enabling the direct deposition of silicon — with varying thickness — on top of glass, must make do with so-called polysilicon, which consists of many small crystals of silicon.

Single-crystal silicon is useful for both optics and electronics, but in polysilicon, there’s a tradeoff between optical and electrical efficiency. Large-crystal polysilicon is efficient at conducting electricity, but the large crystals tend to scatter light, lowering the optical efficiency. Small-crystal polysilicon scatters light less, but is not as good a conductor.

Using the manufacturing facilities at SUNY-Albany’s Colleges for Nanoscale Sciences and Engineering, the researchers tried out a series of recipes for polysilicon deposition, varying the type of raw silicon used, processing temperatures and times, until they found one that offered a good tradeoff between electronic and optical properties.